Superatoms. Группа авторов
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Название: Superatoms
Автор: Группа авторов
Издательство: John Wiley & Sons Limited
Жанр: Химия
isbn: 9781119619567
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Figure 2.30 Geometry of B12(CN)122− with Ih symmetry.
Source: Jena and Sun [1]. © American Chemical Society.
Dianions of organic molecules such as C6H6 and C8H8 can also be stabilized by replacing H with BO and CN superhalogens and two of the carbon atoms by boron. The resulting B2C4X6 2− (X = H, BO, CN) clusters satisfy both the aromaticity rule and octet rule. Chen et al. [54] computed the geometries of neutral, monoanion, and dianions of B2C4X6 (X = H, BO2, CN) clusters. These are given in Figure 2.31; the electron affinity and vertical detachment energies associated with the addition of the first and second electron are given in Table 2.3. Note that the dianions of all molecules are stable, once H is replaced by CN and BO. Similarly, TiC12(CN)12 2− that satisfies both the 18‐electron rule and aromaticity rule is also stable with the second electron bound by 0.76 eV.
Figure 2.31 Optimized geometries of (a) B2C4H60,1−,2−, (b) B2C4(BO)60,1−,2−, (c) B2C4(CN)60,1−,2−, (d) C8(CN)60,1−,2−, (e) C8H80,1−,2−, (f) C8(BO)80,1−,2−, and (g) C8(CN)80,1−,2− clusters. Gray, pink, brown, and red spheres stand for C, N, B, and O atoms, respectively.
Source: Chen et al. [54]. © American Chemical Society.
Table 2.3 First and second electron affinity (EAs) and vertical detachment energy (VDEs) of the studied aromatic molecules.
| Clusters | FVDE | FEA | SVDE | SEA |
|---|---|---|---|---|
| B2C4H6 | 2.01 | 1.90 | −2.68 | −3.27 |
| B2C4(BO)6 | 6.93 | 6.11 | 1.71 | 1.58 |
| B2C4(CN)6 | 5.47 | 5.38 | 1.59 | 1.12 |
| C8(CN)6 | 5.41 | 5.19 | 0.83 | 0.81 |
| C8H8 | 1.47 | 0.82 | −3.35 | −3.49 |
| C8(BO)8 | 5.03 | 4.53 | 1.70 | 1.21 |
| C8(CN)8 | 5.00 | 4.64 | 1.54 | 1.21 |
FEA, first electron affinity; SEA, second electron affinity; and SVDE, second vertical detachment energy.
In some cases, the ligands can play a significant role that can even dominate over the electron‐counting rule. Consider, for example, manganocene, Mn(C5H5)2. It has 17 electrons with Mn contributing 7 electrons due to its 3d 5 4s 2 configuration and each C5H5 contributing 5 electrons. Thus, Mn(C5H5)2 − should be very stable due to the 18‐electron rule and its electron affinity should be high. Instead, its electron affinity is only 0.28 eV [101]. Furthermore, Mn(C5H5)2 − should be unstable against electron emission not only because it is a 19‐electron system but also because the two extra electrons will repel. This is indeed the case. However, if H is replaced by CN or BO, the corresponding Mn[C5(BO)5]2 and Mn[C5(CN)5]2 2− are superhalogens with electron affinities of 4.85 and 4.78 eV, respectively. In addition, Mn[C5(BO)5]2 2− and Mn[C5(CN)5]2 2−, which are 19‐electron systems, are stable with a second electron affinity of 0.38 and 0.7 eV, respectively.
2.3.3 Trianions
Unlike the studies of dianions, work on trianions is rather scarce. One of the early studies of the trianions was due to Compton and coworkers who reported the mass spectra of (C60)2(CN)5 3− and (C60)2(CN)7 3− [102]. Note that observation of species in mass spectra does not necessarily mean that the trianions are stable, but simply that they exist only within experimental conditions. Many metastable multiply charged ions are observed due to repulsive Coulomb barrier. Indeed, (C60)2(CN)5 3− is metastable against autodetachment of the third electron. Cederbaum and coworkers [103] examined the stability of a number of trianions and found that the best candidate, B(C2CO2)3 3−, is thermodynamically unstable against electron emission by −0.4 eV.
Following the discovery of colossal stability of the B12(CN)12 2−, Zhao et al. [104] calculated the optimized geometry and total energy of BeB11(CN)12 in neutral, monoanion, dianion, and trianion form. Note that with Be replacing a B atom, an additional electron will be required to satisfy the Wade‐Mingos rule and the octet rule, simultaneously. In Figure 2.32 we show the geometry, thermal stability at 800K, and electronic structure of BeB11(CN)12 3−, which is stable against auto‐ejection of the third electron by 2.65 eV. When CN is replaced by BO or SCN, the third electron affinities of the resulting trianions BeB11(BO)12 3− and BeB11(SCN)12 3− are, respectively, 1.30 and 0.59 eV.